Conventionally, there has been widely known a technique for performing speed control when a vehicle passes through a curve, on the basis of curve information obtained from map information stored in a navigation apparatus mounted on the vehicle. The curve information (the shape of the curve) obtained from the map information on the vehicle may differ from the actual shape of the curve, for example, in the case where a road has been renovated after the map information (database) had been created. That is, the reliability of the map information (curve information) on the vehicle is not high at all times.
Japanese Patent Application Laid-Open (kokai) No. H11-211492 describes a technique for acquiring final road information from the result of comparison between first road information obtained from map information of a navigation apparatus and second road information obtained on the basis of a road state detected by an image capturing apparatus mounted on a vehicle. The publication describes that, thanks for this technique, reliable road information (curve information) representing a road ahead of the vehicle can be obtained.
Moreover, the publication describes a technique for capturing a stereo image by use of a stereoscopic camera mounted on the vehicle, computing a distance distribution over the entire image on the basis of a difference in position between two images of the same object in the stereo image and the principle of triangulation, and performing histogram processing on the distance distribution to thereby obtain the second road information.
However, in the case where curve information is obtained through image processing as described above, although satisfactory accuracy can be ensured for information representing a curve ahead of the vehicle and located near the vehicle, the accuracy lowers for information representing a curve ahead of the vehicle and located away from the vehicle.